deduplication and encryption.
duplicity (another rsync based one) has deduplication but it's based on directory and file names. If I run filebot on my plex movie directory or take another stab at organizing my music library, rsync, rsnapshot and duplicity will see any files with their names changed as new files. obnam/attic will see the movie/mp3 data blocks are already in the repository and just add a reference to them. As I rename/reorganize regularly this is a huge difference. They've both implemented what duplicity has plans to implement http://duplicity.nongnu.org/new_format.html
encryption is built into obnam/attic, obviously if you're backing to a remote site or taking a disc offsite for safety you'll want to have your backup encrypted. it's not built into rsync or rsnapshot, though it is in duplicity.
so far between the two, obnam has much better memory management than attic so is probably better suited. I'm going to keep using attic for a few weeks though to get a good sense of it.